A gene for chloramphenicol-resistance (cam), derived from an R-factor, is present on a sequence of DNA which is translocatable to diverse replicons (e.g. coliphages P1 and lambda and the Escherichia coli chromosome itself) in the absence of known recombinational functions or sequence homology. The basis of this extraordinary recombinational activity is being studied by genetic analysis and electron microscopy of heteroduplex DNA. A second feature of this "translocon" is that it give rise to deletions at high frequency resulting in the loss of the cam gene from the sequence. Some of the deletions are wholly within the translocon while others extend into the phage DNA. Particular attention is being given to a phage which does not lose the cam gene at high frequency. This phage is deleted for a portion of the phage chromosome adjacent to the translocon. The extent to which the deletion extends into the translocon is being studied.